The invention relates to a machine as defined in the preamble of claim 1.
Although machines of this kind, in the first place, are intended for grinding teeth of circular saw blades they also are suitable, in principle, for machining saw blades of band saws and gang saws as well as milling cutters and the like. Moreover, the work likewise may be done by way of electric discharge machining, using one or more rotating spark erosion discs. Accordingly, the instant invention is not limited to the grinding of teeth of circular saw blades.
Regardless of the general configuration and purpose of the workpieces to be machined and irrespective of the kind of machining to be accomplished, an initial dimension of each individual workpiece must be ascertained prior to beginning the work in order to be able to properly position the workpiece for the machining operation. In the case of circular saw blades, for instance, their diameter must be determined. The machining position envisaged in the case of a machine of the type in question usually is the position at which the tip of the cutting tooth to be machined lies on that axis of the machine which is defined as pivot axis. The tip of the tooth is understood to be a sharp edge defined by the intersection of a tooth face (cutting surface) and the back of the tooth, also called tooth flank (rake).
In order to be able to measure with sufficient accuracy the initial dimension of the workpiece essential for the machining process, this being the diameter in case of circular saw blades, by using the means available on a machine of the generic kind in question, the tip of at least one cutting tooth must be scanned while the corresponding tooth face is in the position it is desired to have. This desired position is determined by the tooth face lying in a plane which includes the pivot axis of the machine.
In a known machine of the type mentioned (DE 196 30 057 C1) the workpiece slide, including a workpiece to be machined having been donned on the workpiece support thereof, is displaced under numerical control along the workpiece slide guide means from a loading position to a position at which a tooth tip has approached the pivot axis of the machine. To be able to accomplish that, it is necessary for the essential magnitude of movement of the workpiece slidexe2x80x94the diameter of a circular saw bladexe2x80x94to be known at least approximately. That requirement is met when workpieces are to be machined which either are new or which had the same dimensions when new and, in the meantime, due merely to resharpening of their teeth, suffered changes which, although varying, remain within certain limits. Under these conditions a certain distance can be preset for a plurality of workpieces to be machined one after the other, the distance defining the displacement of the workpiece slide from its loading position into a position at which the advance means can engage in a tooth gap in order to push the adjacent tooth face into the desired position at which the feeler means, by scanning the corresponding tooth tip, can precisely measure the spacing thereof from the workpiece support, in other words the radius in case of a circular saw blade.
Difficulties are encountered, however, if the known machine of the generic kind in question is to be used for successively machining workpieces of different dimensions, for example, circular saw blades which have different diameters and possibly also different pitches. It would be a very expensive proposition to keep workpieces stored in a magazine in such orientation that the mere donning of each individual workpiece on the workpiece support offers adequate certainty that a tooth tip will result in a sufficiently well aligned position with respect to the pivot axis of the machine so that it can be scanned with satisfactory accuracy by the feeler means when the workpiece slide is pushed ahead. If there is no provision for such preorientation of the workpiece then there is the risk, with the known machine of the generic kind in question, that its feeler means, when moved into measuring position, will enter in a tooth gap and consequently provide an erroneous result of measurement, simulating a smaller workpiece than actually present, for example, a circular saw blade of smaller diameter than the actual one. If the machine were to start operating under these circumstances damages to the workpiece and possibly also parts of the machinery would be unavoidable. Therefore, it was believed to be indispensable up to now to provide the known machine of the generic type in question beforehand with the necessary input of relevant dimensions of the workpieces to be treated so that each individual workpiece donned on the workpiece support will be moved, by numerically controlled shifting of the workpiece slide, into a position at which the advance means can become effective in the way described in order to orient a cutting tooth such that it can be scanned properly by the feeler means.
It is the object of the invention to devise a machine for machining workpieces which have cutting teeth, especially saw blades, such that the machine can receive workpieces of widely differing relevant dimensions in random sequence, without any preorientation, and yet machine them fully automatically.
The object is met, in accordance with the invention, by the features of claim 1.
The design according to the invention of the feeler means makes sure that the frontmost cutting tooth in the direction of movement of the workpiece slide from its loading position towards the pivot axis of the machine will trigger a signal as soon as the tip of this cutting tooth reaches the barrier, even if this tooth tip does not face the pivot axis. In this manner a first dimension is obtained which is characteristic of the workpiece because the respective position of the workpiece slide and the location of the barrier are known to the numerical control means. Normally, the first value thus measured on the workpiece still differs too much from the actually relevant dimension of the workpiece, this being the diameter in the case of a circular saw blade, for any sophisticated machining of the workpiece to be initiated on the basis of this first measured value alone. In most cases, therefore, the first measured value needs some correction to be made. The amount of correction, however, can be determined easily as soon as the advance means has become effective in per se known manner, pushing a tooth face into the desired position at which it lies in a plane which includes the pivot axis of the machine. This tooth face need not be the one of the same cutting tooth of which the tip was sensed originally.
Different ways and means can be applied to sense or scan the tooth tip of the cutting tooth whose tooth face has been moved into the desired position by the advance means. One possibility is to again use the same feeler means which carried out the first measurement in order to find out the correction which is required of the value measured originally. Yet the feeler means must be of more complicated structure to be able to fulfill this task than is required for just obtaining the result of the first measurement.
In general, therefore, it is preferred to provide a separate correcting means, apart from the feeler means, for the renewed scanning of the workpiece and for generating correction signals. This means may comprise a feeler provided purely as such and for no other purpose.
In a particularly advantageous embodiment of the invention, however, the tool itself, provided for machining the workpieces, forms part of the correcting means. Thus, for example, a grinding or eroding disc with which the machine is equipped anyway is used for the renewed sensing of the workpiece. It may serve in different ways as a signal generator, such as by measuring vibrations caused by the workpiece being touched by the tool while the latter rotates.
Another way of obtaining the second measured value is by making the tool an electrically conductive member of an electric circuit which is closed through the workpiece.
There are various possible embodiments of the barrier of the first feeler means as well. For example, it may be a known type of light barrier. In consideration of the fact, however, that a risk of contamination is to be expected in the wider or narrower range around the tool, it is generally preferred to provide a barrier in the form of a physical obstacle which is moved into a sensing position upon request only, while it remains in a protected inoperative position during any machining operation on the workpiece.
A structure as recited in claim 5 is advantageous especially if the barrier is embodied by a physical obstacle since that requires more space than a light barrier.
Further advantageous features may be gathered from claim 6.